Pumps of this type have a very simple gearing, since the pump stroke is obtained by the inclination between the crank axis and piston axis. The crank and piston are typically connected by a connecting rod. During the course of one rotation of the crank, the angles which the connecting rod defines on the one hand with the crank and on the other hand with the piston change. For this reason, in conventional pumps the connecting rod is pivotally connected with the crank and the piston. The joints are susceptible to wear, which can quickly result in play between the connecting rod and its pivot points, which produces excessive noise and also changes the conveying characteristics. Either good lubrication must be provided or else a shorter lifetime must be accepted in such pumps.
Pumps with rotatable pistons have the advantage that they can be constructed as control piston pumps, in which the piston acts as a rotary slide valve. Such pumps have a minimum of movable parts and are extraordinarily robust. In order to assure the rotary slide valve function, the joint between the connecting rod and the piston must be constructed so that angle changes between the connecting rod and piston are possible, but so that the piston is driven for rotary movement with as little play as possible. Through this, special demands are made on the joint.
A basic purpose of the invention is to provide a piston pump of the above-mentioned type in which movements which cause sliding friction are substantially avoided.